1. Field of the Invention
The invention relates to a driving method of a light emitting device comprising a plurality of pixels each of which includes a light emitting element and a means for supplying a current to the light emitting element. The invention further relates to a light emitting device.
2. Description of the Related Art
Emitting light by itself, a light emitting element has a high visibility. Further, since it requires no backlight, a display device using the light emitting element can be easily reduced in thickness and the viewing angle thereof is not restricted. Therefore, the light emitting device using a light emitting element, which attracts attention as an alternative display device to a CRT or an LCD, has been developed for practical use. The light emitting device can be classified into a passive matrix device and an active matrix device. In the active matrix device, a current supply to a light emitting element can be maintained to some extent after a video signal input. Thus, the active matrix device can be flexibly applied to a large panel with high definition and it is expected to become the mainstream in the future. Specifically, each manufacturer offers a different pixel configuration of the active matrix light emitting device, and adopts various technical measures. Though, in general, each pixel comprises at least a light emitting element, a transistor for controlling a video signal input to the pixel, and a transistor for supplying a current to the light emitting element.
For a transistor provided in each pixel of a light emitting device, a thin film transistor (TFT) whose active layer is formed of a thin semiconductor film is mainly employed. Among the TFTs, a TFT using an amorphous semiconductor or a semi-amorphous semiconductor (microcrystalline semiconductor) has the advantage that the cost and the yield can be improved as compared with a TFT using a polycrystalline semiconductor because of fewer manufacturing steps. In addition, such a TFT requires no crystallization step after forming a semiconductor film, therefore, it can be used for forming a large panel with relative ease.
A problem in practical use of a light emitting device is luminance decay of a light emitting element with the degradation of an electro luminescent material. A degradation level of an electro luminescent material depends on the amount of light emitting time and the amount of current flowing. Accordingly, when the gray level differs in each pixel depending on a displayed image, the degradation level of a light emitting element differs in each pixel as well, leading to variations in luminance. In order to suppress such luminance decay, in the following Patent Document 1, a transistor for controlling a current supplied to a light emitting element operates in a saturation region, and a drain current is thus kept constant when the transistor is ON regardless of the degradation of an electro luminescent layer.
Patent Document 1
    Japanese Patent Laid-Open No. 2002-108285